1. Field of the Invention
This invention relates in general to solid state image sensors, and more particularly, to a novel electrode structure for the image sensing portion of a charged coupled device image sensor suitable for use as a color image sensor.
2. Description of the Problem
A charge coupled device (CCD) comprises an arrangement of adjacent metal-insulator-semiconductor (MIS) capacitors on a common semiconductor substrate. A proper voltage applied to the electrode of an MIS capacitor repels majority carriers in the region of the substrate beneath the electrode, thereby creating a potential well, momemtarily depleted of free carriers. Minority carriers introduced into such a potential well can be moved about in the substrate, from one MIS capacitor to an adjacent one by properly manipulating the voltages applied to the capacitor electrodes, called "transfer" electrodes in a CCD. If a photon is absorbed in the semiconductor substrate of a CCD to produce an electron-hole pair near the depletion region of a propery biased MIS capacitor, the minority carrier will be drawn to the depletion region and held in the potential well there. A potential well in a CCD can thus accumulate charge proportional to the total amount of light incident on a portion of the CCD.
It is well known to use a CCD array as an image sensor. FIG. 1 shows a prior art CCD image sensor having a frame transfer type readout organization. The image sensor comprises an image sensing array 10 which produces an imagewise pattern of photocharge in response to imagewise illumination, a storage array 12, which receives the imagewise charge pattern from sensing array 10 and temporarily stores it during readout, and a readout register 14 which transfers the imagewise charge pattern, line-by-line, from storage array 12 to an output diode 16. The image sensing array comprises a plurality of CCD shift registers 18, arranged side-by-side, and having transfer electrodes 20. The device illustrated in FIG. 3 is a three-phase device having three sets of interdigitated transfer electrodes to which three-phase transfer signals P.sub.1, P.sub.2, and P.sub.3 are applied. A plurality of image sensing sites 22 are defined on the image sensing array 10 by properly setting the voltage on transfer electrodes 20.
Similarly, storage array 12 comprises a plurality of CCD shift registers extending from registers 18. Storage array 12 and output register 14 are shielded from light as indicated by the stippling in FIG. 1.
A CCD image sensing array can be made color sensitive by placing an array of color filters over the image sensing sites 22. Ideally, for such color image sensing applications, the CCD image sensor should have substantially uniform spectral response across the visible region of the spectrum. In the simplest early CCD image sensors, the transfer electrodes were patterned from a thin layer of metal (e.g. aluminum) and were therefore opaque to visible light. Light reached the semiconductor substrate through small gaps between the electrodes. Although this arrangement resulted in poor light collecting efficiency, since most of the light reaching the device was reflected from or absorbed by the aluminum electrodes, the spectral sensitivity in the visible region was very good. Unfortunately, the gaps between the electrodes were prone to shorting, and the presence of the gaps produced "fringing" fields that reduced the transfer efficiency of the CCD. In an effort to remedy the problems caused by gaps between electrodes, and to improve the light collecting efficiency of CCD image sensors, semitransparent electrodes of highly doped polysilicon were employed.
A portion of an image sensing array constructed with polysilicon electrodes will now be described with reference to FIGS. 2 and 3. A semiconductor substrate 22, e.g. lightly doped monocrystalline silicon, is covered with an insulating layer 24 of SiO.sub.2. Three sets of transfer electrodes, 26, 28 and 30, are formed on the insulating layer by forming respective layers of highly doped polysilicon and patterning the polysilicon using standard photolithographic techniques. Each set of transfer electrodes is common to all the CCD shift registers 18 (see FIG. 1). Highly doped channel stopping regions 32 separate and define adjacent CCD shift registers. The polysilicon electrodes slightly overlap, and are electrically insulated from each other by layers 34 of SiO.sub.2 grown on each set of transfer electrodes after patterning. The set of electrodes are supplied with three-phase clock signals P.sub.1, P.sub.2, and P.sub.3, respectively. During exposure, the voltages on the sets of transfer electrodes are held such that a potential well 35 is formed under electrodes 26 and 30 (see FIG. 3). An image sensing site 22 is thereby defined between the midpoints of adjacent channel stopping regions 32 and the midpoints of adjacent phase-two transfer electrodes 28. Since the transfer electrodes overlap, the problems associated with gaps between electrodes are eliminated, and since polysilicon is semitransparent, the entire surface area of the device is used to collect light. The light collecting efficiency of the polysilicon electrode device is greatly improved over the metal electrode device.
Unfortunately, however, polysilicon does not transmit the visible spectrum uniformly, absorbing more strongly in the blue region of the spectrum (below about 450 nm). Consequently, the blue response of the polysilicon electrode sensor is marginal for use as a color image sensor. Furthermore, since the amount of blue light absorbed by a polysilicon electrode is a strong function of the thickness of the polysilicon, the blue response is much lower in the areas where the electrodes overlap, making the spectral response of the sensor nonuniform from place to place on the surface of the sensor. This causes a problem when color filters are applied to the sensor, since a slight misalignment of the location of the filters over the image sensing sites can cause large variations in the spectral sensitivity of the sites.
A number of approaches have been proposed for improving the spectral response (e.g. the blue response) in CCD image sensors, while maintaining relatively high light gathering efficiency. For example, U.S. Pat. No. 4,141,024 issued Feb. 20, 1979 to Kano et al, teaches forming partial gaps between adjacent polysilicon electrodes, where light may enter the substrate more directly. The gaps are disposed partially over the channel stopping regions, thereby reducing the effects due to fringing fields, however the spectral response still varies across an image sensing site.
It has also been suggested that conductive transparent metal oxide, with uniform transmission throughout the visible spectrum, be employed in CCD image sensors to form the transfer electrodes. (See U.S. Pat. No. 3,941,630 issued Mar. 2, 1976 to Larrabee). Unfortunately, transparent metal oxide is difficult to pattern, and the processes required to pattern it are not naturally compatible with conventional silicon processing techniques. In the example described in the Larrabee patent, gaps are left between the transparent metal oxide electrodes, reminiscent of the problem causing gaps in the early aluminum electrode CCD's. Thus, the problem faced by the present inventors was to provide a CCD image sensor having substantially uniform spectral sensitivity in the visible region of the spectrum, relatively high light gathering efficiency, and uniform spectral response across the image sensing sites of the sensor.